In countries, such as Poland, neutral admittance protection has become a common earth fault protection function in efforts to provide better effectiveness in earth fault detection when compared to the traditional residual current based earth fault protection in unearthed and compensated distribution networks.
In “Admittance criteria for earth fault detection in substation automation systems in Polish distribution power networks”, J. Lorenc et. al, CIRED 97, Birmingham, June 1997, discloses examples of the implementation of the neutral admittance based earth fault protection and mentions that until mid-1996 over 2000 neutral admittance protection systems have been installed in Poland.
Neutral admittance protection is based on evaluating the quotient Y0=3Ī0/Ū0, i.e. neutral admittance of the network, and comparing the result with operating boundaries in an admittance plane. Residual current 3Ī0 can be measured with a cable core transformer and residual voltage Ū0 is measured from open-delta connected tertiaries of single-pole isolated voltage transformers.
The admittance protection found in existing protection relay terminals can specify that the user select the operation criteria from several possibilities such as 1) over-admittance, 2) over-conductance (non-directional or forward/reverse directional) without/with tilt, 3) over-susceptance (non-directional or forward/reverse directional) without/with tilt or any combination of criteria 1 through 3 (symmetrical around the origin) as desired. FIG. 1 illustrates examples of operation characteristics of existing admittance protection functions presented on an admittance plane (B is susceptance and G is conductance). The shaded area in each of the operation characteristics determines the normal or non-operation area such that, if the neutral admittance is within this area, the protection does not operate and, if the neutral admittance is outside this area, then the protection operates. For example, as shown in FIG. 1, the over-admittance operation characteristic can be defined by setting an absolute value Yset of admittance Y, which defines a circle on the admittance plane as shown. The over-conductance operation characteristic can be defined by lower (−Gset) and upper (+Gset) conductance settings and a tilt may be further set with an angle setting α as shown. In a similar manner, the over-susceptance operation characteristic can be defined by lower (−Bset) and upper (+Bset) susceptance settings and a tilt can be further set with an angle setting α. It should be noted that in the over-conductance and over-susceptance characteristics illustrated in FIG. 1, the shaded non-operation area and the lines defining them have been shown only partly, for the sake of clarity. In addition, different combinations of the operation characteristics can be formed by combining the settings such that, e.g., both the over-conductance and over-susceptance settings are applied at the same time.
In existing solutions, the operation characteristic to be used depends on the network neutral point treatment. For example, in isolated networks the over-susceptance criteria should be applied. In compensated networks it is advised to use the over-conductance based criteria. This means that a relay terminal with admittance protection functionality requires many settings which need to be set according to the network properties. The many settings and possibly difficult setting calculation procedures are a problem with such existing solutions.